MUC1 is a type of mucin glycoprotein. It comprises the core protein coded for by the MUC1 gene (MUC1) and numerous sugar chains that are bonded to the core-protein by O-type sugar chain bonds. Mucin comes in the form of secretory mucin that is produced by epithelial cells and the like, and membrane-bound mucin that comprises a hydrophobic transmembrane portion and is bound to the cellular membrane. MUC1 is a membrane-bound mucin of epithelial cells that is present in normal cells on the distal end surface of mammary gland cells and in milk fat droplets, and in a number of glandular epithelial cells, such as in the pancreas and kidneys (Nonpatent Reference 1). Its molecular size is greater than or equal to 400 kDa, of which 50% is comprised of O-bond-type sugar chains. This glycoprotein is comprised of a short N-terminal region, a central region comprised of tandem repeats 25% of which are accounted for by amino acids having hydroxyl groups, a transmembrane region comprised of 31 amino acids, and a short C-terminal region on the cytoplasm side. The extracellular region containing the central region is severed and released under various conditions. Each tandem repeat (tandem unit) is comprised of 20 amino acids (Pro-Asp-Thr-Arg-Pro-Ala-Pro-Gly-Ser-Thr-Ala-Pro-Pro-Ala-His-Gly-Val-Thr-Ser-Ala) having five sites that can be modified by O-sugar chains. The number of tandem repeats varies between 20 to 125 by allele, so this region is referred to as a variable number of tandem repeats (VNTR). The VNTR region undergoes size polymorphism in which the number of repeats that is expressed varies genetically by individual. Four types of sequence polymorphs are known based on the genetic mutation of specific amino acids. Within this sequence polymorphism, the frequency of the mutation from Pro-Asp-Thr-Arg (PDTR) to Pro-Glu-Ser-Arg (PESR) is high (Nonpatent Reference 2).
MUC1 is known to be overexpressed in many cancers, such as breast cancer, prostate cancer, hepatocellular carcinoma, pancreatic cancer, colon cancer, and ovarian cancer. In particular, overexpression of 90% or more is observed in breast cancer, ovarian cancer, and pancreatic cancer. Further, heightened expression of MUC1 accompanies an adverse prognosis of various cancers, with the concentration of free MUC1 in the blood rising in cancer patients (Nonpatent Reference 1).
The sugar that is initially transferred by O-sugar chain modification to the serine and threonine residues of the VNTR regions of MUC1 with the greatest frequency is GalNAc (sometimes denoted as Tn). As a result, Tn antigen is produced. Although Tn is rarely seen in normal MUC1, it is found in cancer-derived MUC1. Next, sialic acid, galactose, or GlcNAc is added to the Tn, producing sialyl-Tn (STn), core 1 (T), or core 2. When sialic acid is added to core 2, sialyl T (ST) is produced. Other sugar chains are not further added to STn, but GlcNAc and GalNAc are transferred to Tn. In cancer cells, O-type sugar chains are incompletely processed, causing expression of the sugar antigens Tn (GalNAc α-1-Ser/Thr), STn (Sia α2-6 GalNAc α-1-O-Ser/Thr), T (Gal β1-3GalNAc α-1-O-Ser/Thr), core 2 (GlcNAc β1-3GalNAc α1-O-Ser/Thr), ST (Sia α2-3Bal β1-3GalNac α1-O-Ser/THR/Sia α2-6(Gal β1-3)GalNAc α1-O-Ser/Thr) that are common in cancers. As the cancer progresses, the antigen structure (epitope) changes due to different sugar chain modification such as of the five sites in the tandem repeats of MUC1 that are modifiable with 0-sugar chains (Nonpatent Reference 4). Multiple core structures are known for the O-glycan to which GalNAc is initially transferred and numbers have been assigned. The core structures of cores 0, 1, and 2 are given below:    Core 0 (Tn antigen): GalNAc    Core 1 (T antigen): Galβ1-3GalNAc    Core 2: Galβ1-3(GlcNAcβ1-6) GalNAc
The addition of a sugar chain by O-glycosylation of the MUC1 protein plays important roles in the protection of the epithelial cell layer, immune response, cell attachment, and inflammatory response, as well as in cancerization and cancer metastasis. A relation has been reported between the overexpression of MUC1 due to cancerization and the dramatic change of O-glycosylation on the one hand and cancerization and cancerous metastasis on the other. Further, research and development into monoclonal antibodies to MUC1 as diagnostic drugs and treatment drugs for breast cancer and ovarian cancer is advancing (Nonpatent Reference 1). Recently, the interaction between MUC glycoprotein and galectin has been found to be important to cancer progression and metastasis (Nonpatent Reference 5).
Numerous monoclonal antibodies to purified MUC1 and synthetic peptides and glycopeptides derived from MUC1 have been reported (Patent References 1 to 5, Nonpatent References 6 and 7). The minimum sequence recognition of most of these antibodies is thought to lie in the Ala-Pro-Thr-Arg-Pro-Ala-Pro among the peptides in the tandem repeats of MUC1. The threonine that is contained in this sequence is considered to be heavily O-glycosylated, and is thus thought to have an effect on the selectivity and affinity of antibodies binding MUC1. However, for all of the monoclonal antibodies in the above reports, even when a difference based on the presence or absence of sugar chain bonds in the peptides making up the epitope has been identified, no difference has been identified in the sugar chain structure. Thus, cancer cell selectivity has been inadequate. By contrast, the present inventors have prepared antibodies with a high cross-reactivity rate with normal tissue-associated structures relative to the cancer-related structure STn of MUC1 (Patent References 6 and 7). However, even these antibodies have difficulty in accurately recognizing differences in various sugar chain peptide structures in the form of the O-glycosylated core peptides and core peptide structures of MUC1.    Patent Reference 1: JP Patent No. 3698370    Patent Reference 2: JP-A-2002-502621    Patent Reference 3: JP-A-2003-519096    Patent Reference 4: US-A-2006/0292643    Patent Reference 5: JP-A-2010-505775    Patent Reference 6: WO2010/050528    Patent Reference 7: WO2011/135869    Patent Reference 8: JP-A-2006-111618    Nonpatent Reference 1: Beatson et al., Immunotherapy 2: 305-327 (2010)    Nonpatent Reference 2: Engelmann et al., J. Biol. Chem. 276: 27764-27769 (2001)    Nonpatent Reference 3: Bafina et al., Oncogene 29: 2893-2904 (2010)    Nonpatent Reference 4: Clin. Cancer Res. 19: 1981-1983 (2013)    Nonpatent Reference 5: Liu et al., Nature Rev Cancer 5:29-41 (2005)    Nonpatent Reference 6: Danielczyk et al., Cancer Immunol. Immunother. 55: 1337-1347 (2006)    Nonpatent Reference 7: Cao et al., Histochem. Cell Biol. 115:349-356 (2001)    Nonpatent Reference 8: Ohyabu et al., J. Am. Chem. Soc. 131: 17102-17109 (2009)    Nonpatent Reference 9: Matsusita et al., Biochim. Biophy. Acta 1840: 1105-1116 (2014)    Nonpatent Reference 10: Hashimoto et al., Chem. Eur. J. 17: 2393-2404 (2011)    Nonpatent Reference 11: Lu-Gang et al., J. Biol. Chem. 282: 773-781 (2007)Patent References 1 to 8 and nonpatent References 1 to 11 are expressly incorporated herein by reference in its entirety.